1. Field of the Invention
The present invention relates to the supporting of fuel elements within a nuclear reactor and particularly to enhancing the ability of fuel assemblies to withstand seismic loading. More specifically, the present invention relates to high strength nuclear reactor fuel assemblies and to spacer and seismic grids for use therein. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
The functions performed by and the considerations which enter into the design of spacer grids for nuclear reactor fuel assemblies are discussed in detail in U.S. Pat. Nos. 3,607,640 and 3,664,924 issued to Donald M. Krawiec and assigned to the assignee of the present invention. These two patents are incorporated into the present disclosure by reference and may be considered to exemplify the state of the art at the time the invention disclosed herein was conceived.
Fuel assemblies employing the spacer grids of the prior art, for example those of the above-referenced patents, perform admirably under most conditions. The previous spacer grids have usually been fabricated substantially entirely of a zirconium alloy; i.e., zircaloy. The use of annealed zircaloy has been dictated by its desirable combination of mechanical strength, workability and low neutron capture cross-section and the number of spacer grids employed in a single fuel assembly will be minimized to an extent commensurate with structural requirements, in the interest of enhancing reactor operating efficiency. While possessing adequate resistance to buckling under normal operating conditions, laboratory tests have shown that prior art zircaloy spacer grids do not have the mechanical strength required to absorb severe lateral stresses as might be encountered as a result of high seismic loading. While the strength of reactor fuel assembly spacer grids could be increased by the use therein of metals having a greater stiffness than annealed zircaloy, most of such higher strength materials are also characterized by higher neutron capture cross-section when compared to zircaloy and a principal objective in the design of a fuel assembly for a nuclear reactor is to maximize cost and operating efficiency by minimizing neutron capture.
There has previously been reluctance to install nuclear reactors in geographic regions having a history of seismic events of substantial magnitude. This reluctance is, in part, based upon the possible damage to the reactor which might occur as a result of high seismic loading. The type of damage which could conceivably result would be permanent distortion of the zircaloy fuel assembly spacer grids and thus of the fuel assemblies themselves. Any such permanent fuel assembly distortion could preclude or render difficult the removal of fuel assemblies as required during a refueling operation. Laboratory tests have shown that the probable effects of extremely high seismic loading on a fuel assembly would be bending of the guide tubes for the reactor control rods and roll-over of an outer row of the fuel assembly; the outer ring being in part defined by a spacer grid perimeter strip which anchors the spring members which cooperate with the perimeter strip to form the egg-crate type grid. Additional probable effects of high seismic loading include fuel rod support failure and a shifting of the pattern of fuel rods within the fuel assembly to a pattern unfavorable to proper cooling. These deleterious effects would result from fuel assembly to fuel assembly impact under high seismic loading. The possibility of such impact results from the necessity of leaving spacing between the individual fuel assemblies which comprise the reactor core in the interest of facilitating loading and refueling and also to allow for radiation induced growth of the zircaloy components of the fuel assemblies.
To briefly summarize, the fuel assemblies of the prior art have not been suitable for employment in a nuclear reactor intended for use in a location where severe earthquakes are probable because of the possibility of fuel assembly to fuel assembly impact and the lack of sufficient strength of the fuel assembly spacer grids to resist such impacts without undergoing permanent distortion. Previous proposals to enhance spacer grid strength have contemplated the use of materials having greater stiffness and higher neutron capture cross-section when compared to zircaloy. The use of such stiffer materials would impose a significant penalty on reactor operating costs and thus has been an unacceptable alternative to a user such as an electrical utility.